A growing body of research suggests that siblings (especially older siblings) may be an important influence on adolescents'alcohol use, and that their influence is above and beyond the influence of shared genetics, parental socialization, and peer socialization. Despite strong evidence that older sibling alcohol use is linked to younger sibling alcohol use, researchers have been only able to speculate about the mechanisms responsible for these linkages. There is, therefore, a critical need to identify those processes that either positively or negatively influence siblings'decisions regarding alcohol use. In the absence of such information, intervention strategies for reducing negative sibling influences will remain limited. The proposed study examines the extent to which parallel (e.g., modeling, gate-keeping) and differentiation (e.g., sibling de-identification, non-shared environmental factors) influence processes explain the link between older and younger siblings'alcohol use as well as whether family and contextual factors (e.g., sibling relationship qualities, sex constellation of the sibling dyad, parent-child relationships, parenting behaviors) moderate the operation of these influence processes. The sample includes 250 sibling pairs, evenly distributed across the four possible gender composition groups (e.g., older brother, younger sister;older brother, younger brother), and one of their residential parents. Older siblings will be in Grades 11 or 12;younger siblings will be in Grades 9, 10 or 11. Data collection will involve telephone interviews with parents, older and younger siblings about their alcohol and substance related expectancies, beliefs, and consumption patterns, sibling and family relationship qualities, peer relationship qualities, and psychosocial functioning. Hypotheses regarding the nature of the sibling influence processes and moderating function of contextual variables will be examined using a variety of multivariate statistics, including structural equation modeling. PUBLIC HEALTH RELEVANCE: The proposed study examines the multiple avenues by which adolescent siblings'influence each other's decisions about alcohol and substance use. Specifically, we will identify the psychological and behavioral processes that drive sibling similarities and differences with regard to adolescents'alcohol and substance use, as well as the familial and extra-familial contexts that moderate their operation. The identification of these factors is relevant because their discovery will provide new targets for family- based preventions and interventions aimed at curbing adolescent alcohol and substance use.